I have a couple thoughts on common oil leaks. If the intake has been off at some point and they didn't get the rear part of the intake gasket sealed correctly, or if the oil pressure sending unit is leaking, you'll end up with a good leak at the back part of the engine, but I'll look clean up top.
If it's just too much of a mess under there to try and follow the leak up to it's highest point, then you may want to try and wash it all off and then try and follow it up.